Northern sheep producers need to watch for the toxic effects of perennial pastures containing panic and signal grasses.
The warning by the Department of Agriculture and Food follows the recent deaths of about 30 weaner sheep in the Geraldton area.
Department veterinarian Helen Spillman said panic and signal grasses could become toxic when the plants were stressed or growing rapidly.
Dr Spillman said young sheep were most susceptible.
"Cattle are more tolerant but can still be affected when grazing pure stands of panic or signal grass."
Dr Spillman said owners should check sheep, particularly weaners, grazing perennial pastures for signs of toxicity.
"Affected animals develop photosensitisation that leads to swollen, red eyes with thick crusted scabs and droopy and swollen ears," she said.
"The lips, nose and face may also be swollen, with possible hair loss, raw sores or scabs.
"If you see these signs in your flock, contact your veterinarian or district veterinary officer.
"Most animals will recover if removed from the perennial pasture immediately and provided with shade and close access to water and hay."
Dr Spillman said there were several measures sheep owners could take to minimise the risk of toxicity.
"One measure is to graze only cattle or adult sheep for short periods at a high stocking rate. This will reduce selective grazing," she said.
"Another is to feed animals hay for three days before moving them into perennial paddocks and for the first three days while on the perennial pasture.
"Don’t introduce stock onto perennial pastures when they are hungry."
Dr Spillman said toxicity due to panic and signal grasses resulted in liver damage and future grazing of these grasses may quickly reactivate the problem.